The identity of the attackers remained unclear. Russia blamed militant Ukrainian nationalists and the Ukrainian government said the attack near the city of Slovyansk was staged by provocateurs from outside the country.

The armed clash appeared to be the first since an international agreement was reached last week in Geneva to ease tensions in eastern Ukraine, where armed pro-Russia activists have seized government buildings in at least 10 cities.

Ukraine and many in the West fear that such clashes could provide a pretext for Russia to seize more Ukrainian territory. Russia, which annexed the peninsula of Crimea last month, has tens of thousands of troops along its border with Ukraine. Russian officials, including President Vladimir Putin, originally said the troops were there for military exercises, but the president's spokesman yesterday acknowledged that some were there due to instability in eastern Ukraine.

The self-proclaimed mayor of Slovyansk appealed to Russia today to send in peacekeeping troops.

"They are killing our brothers," Vyacheslav Ponomaryov said during a news conference in Slovyansk.

Yuri Zhadobin, who coordinates the pro-Russia unit manning the checkpoint in the village of Bylbasivka, said he was with about 20 men celebrating Easter when unknown men drove up in four vehicles and opened fire.

"We began to shoot back from behind the barricades and we threw Molotov cocktails at them," Zhadobin said. Two of the vehicles caught fire and the attackers fled in the other two, he said.

Some of his men were wounded and one later died in the hospital, he said.

The Ukrainian Interior Ministry released a statement saying one person was killed and three wounded in the shooting.

In Moscow, the Russian Foreign Ministry blamed the clash on the Right Sector, a nationalist Ukrainian group that has supported the interim government in Kiev, the capital, but is not part of it.

But a spokesman for Right Sector, Artyom Skoropatskiy, denied any involvement in the shooting, which he called a provocation staged by Russian special services.

Ukraine's Security Service also called the attack a "cynical provocation" staged from "the outside".

Mr Putin has rejected claims that Russian special forces are directing or encouraging the insurgents. He has said he hopes not to send troops into eastern Ukraine, but he retains the right to intervene if necessary to protect ethnic Russians living here.